This invention is in the field of detection of, and synchronization to, digital signals organized in a sequence of frames transmitted in a medium subject to fading and other changes in propagation characteristics and influences due to receiver and/or transmitter clock frequency drift.
In certain radio broadcast applications, made up of audio signals such as news or music, the content of a radio broadcast is simultaneously distributed to a plurality of locations. In general, such broadcasting can be performed using digital signals embedded in an analog carrier. When using digital signals, the news or music to be transmitted is first digitized with well known analog to digital converters into a series of digital pulses descriptive of the analog signals. Subsequently, the digital pulses are organized into groupings for subsequent transmission. Such a grouping of digital pulses containing digitized news or music make up a frame. Each frame contains digital signals corresponding to an audio signal as well as other digital signals related to the identification of the frame itself and its start, error correction and other functions. Examples of transmission methods using frames for digital audio broadcasting are well known in the art. Examples of digital broadcasting systems are described by R. L. Cupo, M. Sarraf, M. Sharriat and M. Zarrabizadeh in An OFMD All Digital In Band On Channel (IBOC) AM and FM Radio Solution Using the PAC encoder, IEEE Transactions on Broadcasting, Vol 44, No 1, March 1998, pp 22-27; B. W. Kroeger and A. J. Vigil, in Improved IBOC DAB technology for AM and FM Broadcasting, 1996 NAB SBE Conference, Los Angeles, Calif. November 1996; B. W. Kroeger and P. J. Peyla, in Compatibility of FM Hybrid In Band On Channel (IBOC) System Digital Audio Broadcasting, IEEE Transactions on Broadcasting, Vol 43, No 4, December 1997, pp 421-429. Descriptions of OFDM systems are made by W. Y. Zou and Y. Wu in COFDM-An overview, IEEE Transactions on Broadcasting, Vol 41, No 1, March 1995, pp 1-8; J. A. C. Bingham, Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come, IEEE Comm. Mag., May 1990, pp 5-14; as well as J. A. C. Bingham, The Theory and Practice of Modem Design, John Wiley Publishers, New York, 1988, pp 108-121.
Another example of a frame organized, digital transmission system for audio transmission is related to telephone packet switching and is described in the 1990 CCITT interim recommendations on ISDN number I.432, titled B-ISDN User Network Interface Physical Layer Specification and related documents.
In above examples, a string of frame organized digital signals are transmitted over a changing medium, such as air, using a frequency or amplitude modulated carrier. These digital signals are subject to distortion and interference from various probabilistic phenomena such as, for example, Rayleigh fading, attenuation due to precipitation, multipath transmission and others as detailed by K. Bullington in Radio Propagation Fundamentals, Bell System Technical Journal, vol 36, no 3, pp 593-626.
Another interfering phenomenon is clock drift or instability in both the receiver and the transmitter. Small clock frequency shifts, or drift, contribute to phase instability of the carrier emanating from the transmitter, and the corresponding clock mechanism at the receiver, thus adding to the interference induced by external phenomena listed above.
One effect of these probabilistic phenomena is to distort the transmitted digital signal or disable the synchronization mechanism at the receiver thus rendering the frame structure corrupt. This frame corruption presents a problem to the receiver of the frame. If the digital structure of the frame cannot be extracted because of its time distorted content, or phase shift, the receiver cannot correctly extract the digital message within the frame. When the start of a frame cannot be identified, the information contained in the frame is lost, resulting in an undesirable loss of data.
Various methods are known in the art to reduce the impact of data loss related to transmission problems. One approach is to induce retransmission of lost frames, such as described, for example, in open systems interconnection specification X.25 and its progeny. This retransmission of lost frames avoids data loss by redundant retransmission. However, retransmission increases transmission time. To contrast, in music and news broadcast applications, data retransmission is not practical and data loss is characterized by discontinuities in the audio signal, perceived as a decrease in sound quality or permanent data loss. Thus, it is desirable to reduce the occurrence of lost frames so as to increase transmission efficiency in telephone networks and increase audio quality in broadcast systems.
Above problems of frame synchronization are avoided in accordance with the present invention by providing a synchronizing receiver for receiving a digital transmission, the digital transmission composed of a sequence of consecutive bits. The sequence of consecutive bits form a plurality of frames, each of the frames having a frame start. The receiver has a clock for generating pulses at time intervals with respect to a time reference and a counter for counting the time intervals with respect to the time reference thus generating for each of the pulses a count of the time intervals with respect to the time reference. Sampling means, such as A/D converters, for sampling the digital transmission, use the pulses from the clock to extract the digital transmission. A cyclic prefix correlator detects the frame start within the sequence of consecutive bits during a count generated by the counter. This count is indicative of the time interval during which the frame start was detected with respect to the reference. A memory is provided for storing a plurality (typically 36) of counts indicative of the time interval during which the frame start was detected.
A pointer is generated from the counts stored in memory. The pointer is indicative of a projected time interval during which a future frame start is expected to arrive. This projected time interval is computed by using a digital filter. An oscillator responsive to the digital filter generates the pointer.
The digital filter is of the form yn=k0xn+k1xnxe2x88x921+k2ynxe2x88x921, n=0, 1, 2 . . . 35 where n references the frame, yn is the pointer, ynxe2x88x921, is a previous pointer, xn is the count, xnxe2x88x921, is a previous count, and k0=0.003253878916, k1=xe2x88x920.002986, and k2=0.9997325877. A means for adjusting a threshold in response to the absolute value of the difference between said count and said pointer is also provided.
One or more portions of the receiver are implemented using a programmable signal processor.